Process for the preparation of microspherical sintered bodies of hydroxyapatite and a chromatographic packing material comprising the microspherical sintered bodies of hydroxyapatite

ABSTRACT

Microspherical sintered bodies of hydroxyapatite are prepared according to a new specific process wherein a suspension having hydroxyapatite microparticles dispersed in water or an organic solvent or a mixture of water and an organic solvent compatible therewith is atomized into a heating zone maintained above 500° C. whereby spherical agglomerates of hydroxyapatite microparticles formed by drying and solidification of microdroplets of the suspension are sintered while the agglomerates are suspended in the heating zone. Microspherical sintered bodies of hydroxyapatite thus obtained, which may be modified by an optical heat treatment to adjust the surface area of the sintered body and growth of crystals therein, are useful in various fields of industry, for example, as a sorption agent in chromatographic separation.

This application is a continuation, of application Ser. No. 07/321,879filed on Mar. 10, 1989, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to microspherical sintered bodies ofhydroxyapatite and a new process for preparing same as well as achromatographic packing material comprising the microspherical sinteredbodies of hydroxyapatite. More particularly, the present inventionrelates to microspherical sintered bodies of hydroxyapatite which areexcellent in mechanical strength, durability and reproducibility andsuitable as a packing material (sorption agent) for liquidchromatography and a process for preparing same according tospray-firing as a new technical means as well as a chromatographicpacking material comprising such microspherical sintered bodies.

2. Description of the Prior Art

From the past, hydroxyapatite possessing a unique performance as anadsorbent for bio-macromolecules, fluorine ion, heavy metal ions, etc.has been watched as a packing material (sorption agent) for liquidchromatography for the separation and purification of proteins, enzymes,nucleic acids and the like biological substances. In recent years,hydroxyapatite possessing such biocompatibility is utilized also asbio-ceramic materials such as artificial bones and teeth and has abright prospect in this art.

The fact that proteins can be adsorbed to calcium phosphate gel is knownfrom the past. In chromatographic use, however, such gel gives toocompact columns so that the gel fails to give a test solution a flowspeed practical for column chromatography, thus making it difficult touse such gel for a chromatographic purpose. In case of using suchcalcium phosphate gel, therefore, it had to be mixed with a small amountof an auxiliary substance capable of facilitating passage of thesolution through the column, such as Super-Gel before packing. However,such auxiliary substance may adsorb biomacromolecules, thus making theresult complicated. For the above reasons, the calcium phosphate gel isquite undesirable as a packing material for column chromatographicanalysis of bio-macromolecules.

Since the technique for the preparation of hydroxyapatite utilizable forcolumn chromatography was developed by Tiselius et al. [Arch. Biochem.Biophys., 65, 132-155 (1956)], hydroxyapatite for column chromatographicuse has been prepared for a long time up to date according to theirprocesss or various processes wherein their process is modified.According to these processes, however, hydroxyapatite is obtained in theform of plate-like crystals or agglomerates of microcrystals and so hassuch a shortcoming that it is inferior in mechanical strength and tendsto be destroyed during the packing operation and measurement. Thus,chromatographic characteristics of hydroxyapatite are changed accordingto the packing method used or in the course of the measurement so thatproblems arise in durability of the packing material and in trustfulnessof the measurement.

In recent years, a process for producing microspherical hydroxyapatitewas proposed to overcome the above mentioned shortcoming, utilizing theso-called spray-drying method which is widely used for manufacturinggranules of a powdery substance (Japanese Laid-open Patent Appln. Nos.Sho. 62-206445 and 62-230607). According to the process disclosed inJapanese Laid-open Patent Appln. No. 62-206445, microcrystals ofhydroxyapatite having a diameter of less than 1 μm as primary particlesare physically coagulated by spray drying to form substantiallyspherical particles of 1-10 μm in diameter as second particles. JapaneseLaid-open Patent Appln. No. Sho. 62-230607 discloses a process forpreparing spherical agglomerates of apatite wherein a gelledhydroxyapatite slurry is sprayed into an atmosphere kept at 100°-200° C.to form spherical agglomerates of hydroxyapatite having a diameter of1-10 μm. In case the spherical hydroxyapatite particles obtainedaccording to such prior art processes are subjected to classification byscreening to collect particles of a definite particle size as a packingmaterial for liquid chromatography, the spherical particles tend to bedestroyed because of their poor mechanical strength and will be brokento pieces when packed densely in a column under high pressure.Consequently, the spherical hydroxyapatite particles formed by spraydrying have to be brought to a heat treatment carried out at a hightemperature for a long period of time in order to impart to themmechanical strength sufficient enough to withstand high pressure onpacking. Under such severe heat treatment, however, there arises aproblem that the spherical particles tend to be bonded to one another ina mutually fused state to form partially solid state granules. Thus, theprior art processes involve a number of problems not only in thepreparation of spherical hydroxyapatite particles but also in the use ofthe particles as a packing material for chromatographic purposes. Thus,there was a great demand for developing spherical sintered bodies ofhydroxyapatite particles which are excellent in mechanical strength andchromatographic characteristics and easy in handling as well as aprocess for preparing same.

Under the above mentioned circumstances, one of the present inventorspreviously developed a process for preparing microsphericalhydroxyapatite particles according to a specific spray-pyrolysistechnique (Japanese Laid-open Patent Appln. Nos. Sho. 61-146704 and61-201612) as well as a packing material for chromatographic usecomprised of microspherical hydroxyapatite particles possessing highmechanical strength and a process for preparing same (Japanese Laid-openPatent Appln. No. Sho. 62-67451). In these processes, a calcium compoundand a phosphorus compound have to be dissolved, without permitting theformation of any precipitate, in a solution to be sprayed. In practiceof these processes, therefore, some limitations were necessary inselection and combination of the starting materials.

The processes described just above are generally regarded desirable forthe preparation of microspherical hydroxyapatite particle forchromatographic use. However, there is still room for improving theprocesses to obtain microspherical hydroxyapatite particles withdesirable properties in a simpler operation.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aprocess for the preparation of microspherical sintered bodies ofhydroxyapatite possessing high mechanical strength and chromatographiccharacteristics.

It is another object of the present invention to provide a new processfor the preparation of microspherical sintered bodies of hydroxyapatitein one step from a hydroxyapatite slurry according to a specificspray-firing method.

It is still another object of the present invention to providemicrospherical sintered bodies of hydroxyapatite possessing highmechanical strength, good durability and high loading capacity forbiomacromolecules.

It is further object of the present invention to provide achromatographic packing material comprising the microspherical sinteredbodies of hydroxyapatite.

Other objects, features and advantages of the present invention willbecome apparent more fully from the following description.

DETAILED DESCRIPTION OF THE INVENTION

In order to improve further the processes described above, the presentinventors have continuously made extensive researches on the preparationof hydroxyapatite particles with desirable properties. As a result ofthe researches, it has now been found that spherical sintered bodies ofhydroxyapatite exhibiting high mechanical strength, good durability andexcellent chromatographic characteristics such as loading capacity forprotein and high resolution when used as a packing material forchromatographic separation of biomacromolecules can be prepared in onestep from a hydroxyapatite slurry according to a new process utilizing aspecific spray-firing technique. The present invention has beenaccomplished on the basis of the above finding.

In accordance with the present invention, there is provided a processfor the preparation of microspherical sintered bodies of hydroxyapatitein one step and instantaneously from slurried hydroxyapatite whichprocess comprises atomizing the slurried hydroxyapatite into a heatedatmosphere.

In one embodiment of the process, a suspension having hydroxyapatitemicroparticles dispersed in water or an organic solvent or a mixture ofwater and an organic solvent compatible therewith is atomized into aheating zone maintained above 500° C. whereby spherical agglomerates ofhydroxyapatite microparticles formed by drying and solidification ofmicrodroplets of the suspension are sintered while the agglomerates aresuspended in the heating zone.

In another embodiment of the process, a suspension having hydroxyapatitemicroparticles dispersed in water or an organic solvent or a mixture ofwater and an organic solvent compatible therewith is atomized into aheating zone maintained above 500° C. whereby spherical agglomerates ofhydroxyapatite microparticles formed by drying and solidification ofmicrodroplets of the suspension are sintered while the agglomerates aresuspended in the heating zone, and thereafter the resultantmicrospherical sintered bodies of hydroxyapatite is subjected to a heattreatment.

In accordance with the present invention, there are also providedmicrospherical sintered bodies of hydroxyapatite possessing highmechanical strength, good durability and excellent chromatographiccharacteristics, which are prepared according to the above process.

In accordance with the present invention, there is further provided achromatographic packing material which comprises the microsphericalsintered bodies of hydroxyapatite.

The present invention is featured by the structure and properties of themicrospherical sintered bodies of hydroxyapatite. The structure of themicrospherical sintered bodies is distinguished by sphericalagglomerates in sintered state of plural hydroxyapatite microparticleswherein the individual hydroxyapatite microparticles are mutuallysintered. Thus, the microspherical sintered bodies of hydroxyapatitepossess high mechanical strength good durability and excellentchromatographic characteristics such as high loading capacity forprotein and high resolution as described above. The present invention isalso featured by the use of a specific spray-firing technique forpreparing such microspherical sintered bodies of hydroxyapatite, whichis differentiated from the so-called spray-drying method used in theprior art processes.

According to the process of this invention, a suspension (or slurry) ofhydroxyapatite microparticles in water, an organic solvent or a mixtureof water and an organic solvent compatible therewith is atomized into aheating zone above 500° C. whereby the liquid constituting thesuspension (or slurry) is evaporated to form very small sphericalagglomerates of hydroxyapatite microparticles. These sphericalagglomerates are instantaneously fired while they are suspended in theatmosphere of the heating zone whereby the individual hydroxyapatitemicroparticles constituting the spherical agglomerates are subjected tosintering reaction to form spherical sintered bodies where thehydroxyapatite microparticles are mutually sintered to have a structurewith high mechanical strength.

The above mentioned suspension (or slurry) having hydroxyapatitemicroparticles dispersed in water, an organic solvent or a mixture ofwater and an organic solvent compatible therewith will be referred tohereinafter simply as "the HAP slurry". By the specific term"spray-firing" is meant herein the treatment for the formation of themicrospherical sintered bodies of hydroxyapatite instantaneously in onestep from the HAP slurry by atomizing it into a heating zone above 500°C. to cause instantaneously both of the phenomena (a) evaporation of thesolvent contained in the slurry in the form of microdroplets by dryingand (b) the formation of spherical agglomerates of hydroxyapatitemicroparticles by solidification. The above mentioned sphericalagglomerates of hydroxyapatite microparticles will be referred tohereinafter simply as "the HAP sphere".

In the prior art processes, on the contrary, similar microsphericalsintered bodies of hydroxyapatite are prepared in two steps; the step ofspray-drying an aqueous slurry of hydroxyapatite wherein the aqueousslurry is atomized into a heating zone to effect dehydration and dryingof microdroplets of the slurry for obtaining agglomerates ofhydroxyapatite microparticles and the step of granulating theagglomerates to form spherical granules of hydroxyapatite and heatingthe granules in a heat-resisting container placed in a furnace or thelike heating source to effect sintering of the individual hydroxyapatitemicroparticles constituting the spherical granules. In some cases,drying of the agglomerates of hydroxyapatite microparticles becomesinsufficient only by the spray-drying step. In such cases, a drying stepover a long period of time is required in the prior art processes priorto the heat treatment for sintering. Thus, the prior art processesnecessitate, in addition to the spray-drying operation for granulationof hydroxyapatite microparticles, a sintering treatment of thegranulated hydroxyapatite spheres and a drying treatment providedbetween the spray-drying operation and the sintering treatment forpreparing sintered spherical particles of hydroxyapatite forchromatographic use. Further, such drying and sintering treatments haveto be carried out in the prior art processes separately from thespray-drying operation.

The spray-firing treatment in the process of this invention is, at aglance, similar to the spray-drying treatment in the prior art processesin that the HAP slurry or an aqueous slurry of hydroxyapatitemicroparticles is atomized into a heating zone where thermal energy isimparted to microdroplets of the slurry by using a heated gas or thelike heating means. However, the spray-firing treatmment of the presentinvention is fundamentally different from the conventional spray-dryingtreatment in the following point: A prime object of the conventionalspray-drying resides in dehydration and drying of droplets of an aqueousslurry of hydroxyapatite microparticles wherein the individualmicroparticles are mechanically attached to one another. The temperaturegenerally employed for spray-drying is insufficient to impart thermalenergy sufficient to cause sintering reaction to dry sphericalagglomerates of hydroxyapatite microparticles. Contrary to this, a primeobject of the spray-firing treatment of the present invention is toeffect, in addition to drying of the HAP spheres, a sintering reactionof hydroxyapatite microparticles constituting the HAP spheres. In thespray-firing treatment, therefore, a thermal energy much higher thanthat for the spray-drying treatment is required which must be sufficientenough to attain not only dehydration and drying of the HAP slurry forthe formation of the HAP spheres but also sintering reaction of theindividual hydroxyapatite microparticles constituting the HAP spheres.Thus, a temperature higher than 500° C. far beyond the temperature rangefor spray-drying is required in the heating zone. According to theprocess of this invention, a great advantage is achieved in thatmicrospherical sintered bodies of hydroxyapatite can be obtainedinstantaneously by dehydration and drying of the HAP slurry followed bysintering of the resultant HAP spheres while they are suspended in thespace of the heating zone.

In the process of this invention, discrete HAP spheres are heated whilebeing suspended in the atmosphere of the heating zone so that thepartial pressure of water vapor in the atmosphere surrounding theindividual HAP spheres becomes equal. Further, only the hydroxyapatitemicroparticles constituting the HAP spheres are mutually sintered, thusenabling to prevent occurrence of an undesirable phenomenon that the HAPspheres (or microspherical sintered bodies of hydroxyapatite) aremutually bonded by sintering. As the partial pressure of water vapor inthe atmosphere surrounding the individual HAP spheres is equal, anespecially remarkable merit is achieved in case of using the resultantmicrospherical sintered bodies of hydroxyapatite as a chromatographicpacking material. Hydroxyapatite contains OH groups as crystal water andloses a part of the OH groups by heating. In this case, the degree ofdeficiency in crystal water is influenced by the partial pressure ofwater vapor in the atmosphere of the heating zone. On the other hand,the degree of deficiency in crystal water (or the number of OH groups)in hydroxyapatite crystal is greatly influenced on chromatographiccharacteristics in case of using hydroxyapatite as a packing materialfor chromatography. Accordingly, it is of importance thatcharacteristics of the individual microspherical sintered bodies ofhydroxyapatite, including the degree of deficiency of OH groups, aresame.

In the prior art processes, however, agglomerates of hydroxyapatiteparticles formed and dried according to spray-drying are firstlygranulated and then heated for a long period of time in a heat-resistantcontainer for sintering. In case the spherical hydroxyapatite granulesare placed in the container, the atmosphere surrounding the granules onthe superficial portion is different in partial pressure of water vaporand in easiness in removal of any residual gas contained in the granulesfrom the atmosphere surrounding the granules positioned in the bottomportion, thus permitting occurrence of difference in atmospheresurrounding the individual granules. Such difference apparently causesfluctuation in characteristics of the individual hydroxyapatitemicroparticles. In case the spherical hydroxyapatite granules aresintered in a rotary kiln under rotation or the like heating means so asto make the atmosphere surrounding the individual granules as similar aspossible, the spherical granules having been dried but not yet beensintered are so poor in mechanical strength that the granules may bedestroyed prior to being sintered. In case of the prior art processeswherein the preparation of the spherical sintered bodies ofhydroxyapatite is carried out stepwise, it is difficult to obtain theproduct having satisfactory characteristics. What is more, the firingtreatment in the prior art processes of spherical hydroxyapatitegranules tends to give a product wherein the granules have mutually beensintered as the individual granules are contacted with one another inthe container. Accordingly, the microspherical sintered bodies ofhydroxyapatite obtained in the prior art are not uniform in size and areinconvenient in handling.

In general, hydroxyapatite suitable as a chromatographic packingmaterial is the so-called Ca-deficient type hydroxyapatite wherein theratio of Ca to P, i.e. Ca/P, is smaller than the theoretical ratio. Asthe Ca-deficient type hydroxyapatite is easily decomposed by heating, itis especially important to prevent decomposition of the hydroxyapatiteby maintaining moderately the partial pressure of water vapor in thefiring atmosphere and shortening the heating time. In the process ofthis invention wherein an inflammable organic solvent such as an alcoholis used as a dispersion medium for the preparation of the HAP slurry,the merit of this invention is maximally increased. When the HAP slurryusing an inflammable organic solvent as a dispersion medium thereof isatomized into a heating zone, the solvent was evaporated and burnt byflame in the heating zone whereby the heat of combustion of the organicsolvent can be utilized as a heat source for heating, drying and firingthe HAP slurry. Moreover, water vapor formed by the combustion of theorganic solvent affords a moderate partial pressure of water vapor inthe atmosphere of firing the HAP spheres. Thus, there is no necessity ofexternal supply of partial pressure of water vapor.

According to the process of this invention, the HAP spheres formed byatomization of the HAP slurry are dried and fired within an extremelyshort peirod of time while the HAP spheres are suspended in the heatingzone so that the steps for the preparation of microspherical sinteredbodies of hydroxyapatite and the time required for such preparation canbe minimized, thus bringing about a great industrial merit in thepreparation of hydroxyapatite. Further, the process of this inventionenables to maintain the atmosphere surrounding the individual HAPspheres quite even throughout the firing treatment so thatmicrospherical sintered bodies of hydroxyapatite suitable as achromatographic packing material can be provided.

In practice of the process of this invention, any type of commerciallyavailable hydroxyapatite can be used as a starting material.Alternatively, hydroxyapatite utilizable as a starting material can beprepared as needed according to any desirable method described in therelevant publications. Thus, the method for preparing hydroxyapatite isnot specified in the present invention but the following methods can bementioned as typical examples of the method recommended for preparinghydroxyapatite:

(1) The so-called wet process known, for example, as Tiselius method [A.Tiselius, et al., Arch. Biochem. Biophys., 65, 132-155 (1956)] whereinhydroxyapatite microparticles are precipitated from an aqueous solutionby a direct ionic reaction.

(2) The so-called dry process according to a solid-state reactionbetween a calcium compound and a phosphorus compound.

(3) The hydrothermal synthesis wherein a reaction is carried out betweena calcium compound and a phosphorus compound in the presence of steam athigh temperature and pressure.

A solution containing a precipitate of hydroxyapatite microparticlesformed according to the wet process (1) may directly be used as the HAPslurry.

No particular limitation exists in the particle size of hydroxyapatiteused in the process of this invention. To obtain a stable suspension ofhydroxyapatite wherein hydroxyapatite microparticles are kept in abetter dispersed state, it is desirable that the particle diameter ofhydroxyapatite is 1 μm or less.

The dispersion medium used for the preparation of the HAP slurry isselected from water, an organic solvent and a mixture of water and anorganic solvent compatible therewith. Any kind of organic solvent can beused for the present invention except that the organic solvent shouldnot be viscous or non-combustible. In general, therefore, the organicsolvent is selected from easily combustible, volatile, non-viscous,liquid organic substances. Standing on the above viewpoint, a loweralkanol with 1-8 carbon atoms, a lower alkane with 6-8 carbon atoms, alower dialkyl ketone with 3-8 carbon atoms, an aromatic hydrocarbon with6-8 carbon atoms and a dialkyl ether with 3-8 carbon atoms arepreferable for the present invention. Illustrative of the preferableorganic solvent are, for example, methanol, ethanol, propanol, butanol,pentanol, hexanol, heptanol, octanol, hexane, octane, acetone, methylethyl ketone, diethyl ketone, methyl isobutyl ketone, benzene, toluene,xylene, diethylether, ethyl propyl ether and dipropyl ether. When amixture of water and an organic solvent is used as a dispersion medium,the organic solvent must be compatible with water, without forming aheterogenous phase. Accordingly, the organic solvent in this case ispreferably miscible with water, for example, a lower alkanol such asmethanol or ethanol, a ketone such as acetone or methyl isobutyl ketone.In case a mixture of water and an organic solvent is used as adispersion medium, any of the mixing ratios can be used.

Any of the mixing means such as a stirrer can be used for preparing theHAP slurry. No particular limitation exists in the concentration ofhydroxyapatite in the HAP slurry. The concentration may be variedaccording to the kind of atomizing nozzle used, the diameter of nozzle,the pressure for atomization of the HAP slurry and the viscosity of theHAP slurry as well as the size of the microspherical sintered bodies ofhydroxyapatite to be prepared and the intended use thereof. Mixing ofhydroxyapatite with the dispersion medium by the aid of a mixer, stirreror a homogenizer can be carried out at ambient temperature.

The term "hydroxyapatite" used herein includes, in addition tohydroxyapatite of a stoichiometric composition [Ca₁₀ (PO₄)₆ (OH)₂ ], theso-called Ca-deficient type hydroxyapatite wherein the content ofcalcium is smaller than the theoretical value, a modified hydroxyapatitesynthesized by varying the ratio of calcium to phosphorus, and asubstance consisting substantially of hydroxyapatite with a small amountof other compounds such as tricalcium phosphate. These can be used aloneor in mixture as hydroxyapatite for the present invention.

The HAP slurry thus prepared is atomized into a heating zone maintainedat a temperature above 500° C. Any type of the conventional atomizerswith a proper nozzle, such as those equipped with a pressure nozzle or atwo-fluid nozzle can be used for this purpose.

The term "heating zone" used herein means a confined space with an inletand an outlet maintained at a given temperature above 500° C. by flameformed by combustion of a combustible gas, a gas furnace, an electricfurnace or a stream of a high temperature gas. Other suitable heatsources can also be used for providing the heating zone.

When the HAP slurry is atomized into such heating zone, microdroplets ofthe slurry formed are suddenly heated at a high temperature whereby thedispersion medium contained in the microdroplets is instantaneouslyevaporated and/or burnt, leaving microparticle or hydroxyapatite. Themicroparticles of hydroxyapatite thus formed are agglomerated to formthe HAP spheres which are also heated in the zone while being suspendedtherein whereby the hydroxyapatite microparticles constituting the HAPspheres are subjected to sintering reaction and mutually bonded.

As briefly referred to hereinbefore, remarkable advantages are obtainedin the present invention when a combustible organic solvent or a mixtureof water and an organic solvent which is combustible and miscible withwater is used as a dispersion medium for the HAP slurry. In case the HAPslurry containing such dispersion medium is atomized into the heatingzone maintained above 500° C., the combustible organic solvent containedtherein is instantaneously evaporated and burnt whereby the heat ofcombustion is evolved as high heat energy which can effectively utilizedas a heat source necessary for sintering reaction of the hydroxyapatitemicroparticles in the HAP spheres. In this embodiment, a narrow spacesurrounding the individual HAP spheres can instantaneously be heated toa high temperature so that the heat can effectively be utilized.Accordingly, the preparation of a large amount of the microsphericalsintered bodies of hydroxyapatite per unit time becomes possibleaccording to this embodiment, thus bringing about significant industrialmerits. As the combustion of the organic solvent accompanies thegeneration of water, a moderate partial pressure of water vapor can beimparted to the atmosphere surrounding the HAP spheres when thehydroxyapatite microparticles constituting the HAP spheres are mutuallysintered. Thus, there is no necessity of supplying water vaporexternally which is necessary in the atmosphere when hydroxyapatitecontaining OH groups as crystal water. This is another merit of thisembodiment. It is widely known that reactions for liberation of the OHgroups in hydroxyapatite and for decomposition are accelerated whenwater vapor is absent or insufficient in the atmosphere wherehydroxyapatite is sintered. Thus, the process of this invention whereinwater generated on the combustion of a combustible organic solvent caneffectively be utilized can be said to be a highly desirable sinteringmethod.

Among the heat sources mentioned above and used in the heating zone, agas furnace is desirably used for accelerating evaporation of thedispersion medium, especially containing water. It is of course possibleto use combustion of a gas in the furnace jointly with combustion of theorganic solvent as a heating source. The microspherical sintered bodiesof hydroxyapatite thus obtained can be collected by a proper recoveringmeans such as a cyclone.

The microspherical sintered bodies of hydroxyapatite of this inventionobtained according to the above process are usually from 0.5 μm toseveral ten um in diameter and possess high mechanical strength. Thisproduct can be used as such for chromatography as a packing material(sorption material). Alternatively, the product may be classifiedaccording to a known conventional screening method to obtain a fractionhaving a definite particle diameter, considering its high mechanicalstrength.

If necessary, the product may further be subjected to a heat treatmentconducted at 500°-900° C. for a proper period of time according to theintended purpose, for example, by using a gas furnace, an electricfurnace or the like heating means. This optional heat treatment isuseful for adjusting the surface area of the sintered bodies or thedegree of crystal growth, or for removing any contaminant such as anyremaining gas in the microspherical sintered bodies. As themicrospherical sintered bodies of hydroxyapatite of this invention hassuch a structure that the individual hydroxyapatite microparticles havemutually been sintered, there is no fear of further bonding of thesintered bodies by the optional heat treatment. Thus, the product ofthis invention is very convenient in handling.

The present invention will now be illustrated in more detail by way ofExamples.

EXAMPLE 1 (1) Preparation of microspherical sintered bodies ofhydroxyapatite

Hydroxyapatite microparticles of 0.2 μm or less in diameter synthesizedaccording to the wet process were dispersed in a mixture of water andmethanol (1:1) and the dispersion was vigorously stirred to form a HAPslurry having a concentration of 1 mol/liter. The HAP slurry andcompressed air were supplied at rates of 10 ml/min and 10 liters/min,respectively, to a two-fluid nozzle and then atomized into flame of aburner whereby the methanol in the HAP slurry was evaporated and burnedby the flame. Microdroplets of the HAP slurry formed by atomization wereinstantaneously heated to about 580° C. to form microparticles by theheat of the flame and the heat of combustion of the methanol. Thesemicroparticles were collected by a cyclone and subjected to scanningelectron microscopy and X-ray diffraction analysis. As a result of theanalyses, the microparticles were found to be spherical sintered bodiesof hydroxyapatite having a particle size of 1-16 μm wherein theindividual hydroxyapatite microparticles had been mutually sintered andgrowth of crystals had occurred.

(2) Liquid chromatography

The spherical sintered bodies of hydroxyapatite obtained in the above(1) were classified with the aid of an air-classifier and only thosehaving a diameter of 4-8 μm were collected. The spherical sinteredbodies of hydroxyapatite thus collected were subjected to a heattreatment conducted for 3 hours at 750° C. to obtain a packing materialfor liquid chromatography.

A stainless steel column (8 mm in inner diameter, 100 mm in length) wascharged with the packing material according to a slurry packing methodunder pressure of 300 kg/cm².

Using this column, the measurement of a mixed sample consisting ofbovine serum albumin, lysozyme (egg white) and cytochrome C (horseheart) was repeatedly carried out at a flow rate of 1.0 ml/min to checkreproducibility of the column according to a linear gradient method(from 0.01M to 0.3M in 60 min) using a sodium phosphate buffer solutionhaving a pH value of 6.8. As a result of the measurement repeated 58times in all, the fluctuation coefficient in retention time of eachsample was recorded as 1.30% in case of bovine serum albumin, 0.72% incase of lysozyme and 0.67% (reduced form) or 0.90% (oxidized form) incase of cytochrome C, thus showing extremely high reproducibility. Bythe way, the retention time of each sample in the first and the 58thmeasurements was as shown in the following table:

                  TABLE                                                           ______________________________________                                                         Measurement                                                  Sample             1st      58th                                              ______________________________________                                        Bovine serum albumin                                                                             17.84 min                                                                              17.13 min                                         Lysozyme           27.33 min                                                                              28.06 min                                         Cytochrome C (reduced)                                                                           44.30 min                                                                              45.42 min                                         Cytochrome (oxidized)                                                                            47.16 min                                                                              48.74 min                                         ______________________________________                                    

The number of theoretical plates per column in the first measurementshowed a value as high as 11600 while that in the 58th measurementshowed 11800 which was in agreement with the value in the firstmeasurement within a statistical error. The column permitted no changein the number of theoretical plates even after the use of many times,thus showing a very good durability. The loading capacity of the columnfor protein was checked by adsorbing thereto 40 mg of lysozyme wherebyit was found that all of the lysozyme was adsorbed to the column.

In view of the above results, it was found that the spherical sinteredbodies of hydroxyapatite concerned with this invention were very usefulas a chromatographic packing material.

EXAMPLE 2

Hydroxyapatite microparticles of 0.3 μm or less in diameter weredispersed in ethanol and the dispersion was vigorously stirred to form aHAP slurry having a concentration of 0.5 mol/liter. The HAP slurry wasatomized into flame of a burner in the same manner as described inExample 1 by the aid of a two-liquid nozzle whereby the ethanol in theHAP slurry was evaporated and burned by the flame. Microdroplets of theHAP slurry formed by atomization were instantaneously heated to about1360° C. to form microparticles by the heat of the flame and the heat ofcombustion of the ethanol. These particles were collected by a cycloneand subjected to scanning electron microscopy and X-ray diffractionanalysis. As a result of the analyses, the microparticles were found tobe spherical sintered bodies of hydroxyapatite having a particlediameter of 0.5-14 μm wherein the individual hydroxyapatitemicroparticles had been mutually sintered and growth of crystals hadoccurred.

EXAMPLE 3

Hydroxyapatite microparticles of 0.2 μm or less in diameter weredispersed in water and the dispersion was vigorously stirred to form aHAP slurry having a concentration of 0.3 mol/liter. A distilling flaskwas connected to one end of a tubular furnace in such manner that theair in the furnace heated at 1200° C. might be sucked by an aspirator.The HAP slurry was atomized by the aid of a pressure nozzle into thefurnace at a flow rate of 1 ml/min, and the microparticles formed wascollected in the flask. Microdroplets of the HAP slurry formed byatomization were heated while allowed to pass through the furnace toform microparticles. As a result of scanning electron microscopy andX-ray diffraction analysis of the microparticles, they were found to bemicrospherical sintered bodies of hydroxyapatite having a particlediameter of 0.5-12 μm wherein the individual hydroxyapatitemicroparticles had been mutually sintered and growth of crystals hadoccurred.

It is understood that the preceding representative examples may bevaried within the scope of the present specification, both as to thesubstances to be used and the treating conditions, by one skilled in theart to achieve essentially the same results.

As many apparently widely different embodiments of the present inventionmay be made without departing from the spirit and scope thereof, it isto be construed that the present invention is not limited to thespecific embodiment thereof as illustrated in examples except as definedin the appended claims.

What is claimed is:
 1. A spray firing process for the preparation ofmicrospherical sintered bodies of hydroxyapatite, comprising:atomizinginto a flame a suspension of hydroxyapatite microparticles dispersed inan inflammable organic solvent, or in a mixture of water and aninflammable organic solvent compatible with water, whereby the resultingmicrodroplets of the suspension of said hydroxyapatite are heated by theheat of the combustion of said inflammable organic solvent or saidinflammable organic solvent compatible with water, to instantaneouslyform microspherical sintered bodies of hydroxyapatite; and collectingthe formed microspherical sintered bodies of hydroxyapatite.
 2. A sprayfiring process for the preparation of microspherical sintered bodies ofhydroxyapatite, comprising:atomizing into a flame a suspension havinghydroxyapatite microparticles dispersed in an inflammable organicsolvent, or in a mixture of water and an inflammable organic solventcompatible with water, whereby the resulting microdroplets of thesuspension of the hydroxyapatite are heated by the heat of combustion ofsaid inflammable organic solvent or said inflammable organic solventcompatible with water, to instantaneously form microspherical sinteredbodies of hydroxyapatite; collecting said formed microspherical sinteredbodies of hydroxyapatite; and subjecting said sintered bodies ofhydroxyapatite to heat treatment at a temperature within the range of500°-900° C.
 3. The process according to claim 1 or 2, wherein saidinflammable organic solvent or said inflammable organic solventcompatible with water is selected from the group consisting of methanol,ethanol, and acetone.